The radio tag communication system has a tag and RW apparatuses carry out radio communication. The tag includes a dipole antenna A and a dipole antenna B, and a tag IC generates circularly polarized waves with a phase difference of 90° from the dipole antennas A and B (see Patent Document 1, for example).
The tag, however, includes no power supply. Accordingly, receiving the high-frequency signal transmitted from the RW apparatus with the dipole antennas A and B, the tag rectifies the high-frequency signal and stores the power, and carries out radio communication with the RW apparatus using the power.
Thus, the system has a characteristic in that the communication area between the tag and RW apparatus is limited to a range of a few meters.
Accordingly, a plurality of RW apparatuses must be arranged to implement a communication area beyond a few meters.
In addition, to recognize all the tags in the communication area beyond a few meters, the communication area must be continuous without a break.
In this case, the plurality of RW apparatuses must have communication areas that overlap one another.
Patent Document 1: Japanese patent application laid-open No. 2003-249871 (pp. 12-13 and FIG. 2).
With the foregoing configuration, the conventional radio tag communication system enables, when the communication areas of RW apparatuses A and B overlap, the tag and both the RW apparatuses A and B to carry out radio communication within the overlapping areas. However, the conventional radio tag communication system has a problem of disturbing the radio communication between the tag and the RW apparatus A when the RW apparatus B transmits a signal while the tag carries out the radio communication with the RW apparatus A because the signal from the RW apparatus B causes interference with the signal from the RW apparatus A.
The present invention is implemented to solve the foregoing problem. Therefore it is an object of the present invention to provide a radio tag communication system capable of preventing the interference of the signal even when extending the communication area.